1. Technical Field
This invention relates to metrology instruments and more particularly to an improved control and phase sensitive demodulator circuit used with an AC excited transducer.
2. Background Art
Transducers for sensing movement which require AC excitation and provide a modulated output, such as a linear variable differential transformer or LVDT, are well known in the prior art. An LVDT includes a primary winding, on an insulating spool, which is excited by a constant frequency and amplitude AC voltage. A pair of secondary coils are wound on the spool on each side of and in close proximity to the primary coil. The pair of secondary coils are connected in series opposition so that the two voltages in the secondary circuit are opposite in phase. An elongated movable core is positioned inside of the spool and provides a path for magnetic flux linking the coils. With the secondary coils connected in series opposition and with the moveable core in a null position theoretically there is zero output on the secondary. When the core is moved from the null position, voltage induced in the secondary coil towards which the core is moved increases, while voltage induced in the other secondary coil decreases. The LVDT thus produces a differential voltage which varies linearly with the core displacement from the null position. Motion of the core in the opposite direction produces a similar linear voltage but of an opposite phase.
The movable core can be connected to various devices for indicating changes in parameters such as force, weight, pressure, distance or acceleration. By mechanical connections the various devices cause movement of the core which is indicative of the parameter being measured. Due to its small core mass and low friction the LVDT can accurately follow rapid changes in displacement. The output of the AC excited LVDT transducer is a modulated wave whose envelope accurately portrays the wave form, frequency and amplitudes, of the core movement. The transducer output is demodulated to provide an output corresponding to core movement.
A synchronous demodulator can be used to extract the transducer core position information from the carrier. LVDT error signals result from small quadrature components and other nonideal transformer properties. With the core in a null position the LVDT still has an error signal output. At low LVDT signal levels the error signal is relatively large. By synchronous demodulation all signals not synchronized to the LVDT excitation frequency are averaged to zero. In the April 7, 1981 issue of Electronics pages 121 thru 129, which is herein incorporated by reference, the use of a GAP-01 for synchronous demodulation is described.
There is normally a phase shift between the output of the LVDT and the input excitation. To compensate for this phase shift in prior art signal conditioners a phase shift circuit is provided in parallel with the LVDT, between the excitation source and the demodulator. The phase shift is commonly provided by an RC circuit. An RC circuit however is limited to an ideal phase shift of 90 degrees for a single RC pair. Also the tolerence on inexpensive capacitors is typically five to twenty percent, thus effecting the accuracy of the phase shift. Resistor and capacitor values also change with temperature and this will further effect phase shift accuracy. When a fixed phase shift network is used for a family of LVDTs, each of which has its own specific phase shift, there is a loss of sensitivity due to nonideal matching of the secondary phase with the demodulator phase. While this effect may be small and may not effect linearity in itself, it is disadvantageous in that higher amplifier gain is required to compensate for the lost sensitivity. Mismatch also tends to exaggerate the errors as the phase shift further changes with respect to temperature, time or excitation frequency. The higher amplifier gain can then cause higher drift within the electronics system.
Another approach to the problem is to design an LVDT with a zero degree phase shift at the operating frequency. Designing an LVDT with zero degree phase shift, however, can add significant cost to the manufacturing process and often is difficult to maintain for all different ranges of a family of LVDTs. Ignoring the existence of phase shift can cause linearity problems as the frequency, phase and harmonics of the system change with temperature and time.
Existing LVDT phase sensitive demodulators often suffer performance problems due to several sources such as instability of primary voltage amplitude, instability of the phase shift of the reference voltage to the demodulator, improper matching of the demodulator reference voltage phase to the phase of the secondary output signal from the LVDT, harmonics present in the primary voltage source, and instability of the excitation frequency.